The present invention relates to image signal recording apparatus and methods, for example, in which a still image is recorded digitally in a PCM audio recording area of a magnetic tape by an 8 mm video tape recorder.
Several known apparatus are available for recording still video pictures on recording media, such as magnetic media. For example, an electronic still camera records video images on a still video floppy disk. Another such device is the 8 mm video tape recorder (VTR) which is provided with a built-in camera.
FIG. 1 illustrates a standard format employed by an 8 mm VTR for recording signals on a magnetic tape 170. The tape 170 is wound circumferentially about a rotary head drum of the 8 mm VTR to an angular extent of 211 degrees. The head drum includes a pair of diametrically opposed rotary heads which scan the tape 170 along helically extending paths in order to form obliquely extending recording tracks 171 on the tape 170. Each of the tracks 171 includes a video area 172 extending for 180 degrees of the 211 degree extent of each track 171. The VTR records frequency multiplexed signals in the video area 172 including frequency modulated luminance signals, subcarrier chrominance signals which have been shifted to a lower frequency band, frequency modulated audio signals and a plurality of pilot signals provided for tracking control. The remaining portion of each track 171 extending for approximately 30 degrees is set aside for recording pulse code modulated (PCM) audio signals digitized at 8 bits per sample and processed by non-linear quantization. The pulse code modulated audio signals are cross-interleaved for error correction and synchronizing signals, parities and identification signals are added thereto prior to recording. A cue track 174 is recorded longitudinally on a first lateral edge of the tape 170 by a fixed head, and an audio track 175 is recorded longitudinally by another fixed head along the opposite lateral edge of the tape 170. The cue track 174 as well as the audio track 175 may be used to record cue or editing signals for locating previously recorded information.
Japanese Patent Publication No. 58-164383 (1983) describes a technique for recording still pictures such as letter or character information by after-recording in the PCM audio area 173 to superimpose titles, casts or explanatory text on moving pictures recorded in the video area 172.
If it is desired to record a high resolution still picture with the use of the 8 mm VTR by recording digitally in the PCM audio area of the tape, it is necessary to employ a relatively high sampling frequency and a suitable imaging device such as a CCD image sensor having a relatively large number of photoelectric converting elements. Such apparatus, referred to hereinafter as an "upper level system", is relatively complex and costly. Less expensive 8 mm VTR's, referred to hereinafter as "lower level systems", employ CCD image sensors having a relatively smaller number of photoelectric converting elements so that a lower sampling frequency is used thereby.
Due to the disparity in data rates between the upper and lower level systems, it is difficult to realize interchangeability of recordings so that a tape recorded with the use of an upper level system may not be reproduced by a lower level system without processing the recorded signals, for example, by interpolation in order to derive picture data having the necessary low data rate. An example of such an interpolation process is illustrated in FIG. 2 wherein the picture data a.sub.1, a.sub.2, . . . a.sub.1152 of each line of a first video signal are recorded by an upper level system and are to be combined by interpolation to form picture data b.sub.1, b.sub.2, . . . b.sub.768 in each line of a converted video signal having a data rate which is compatible with that of a lower level system. In the example of FIG. 2, therefore, the ratio of the data rate of the upper level system to that of the lower level system is 6:4. The interpolation process is carried out by combining selected pairs of the data a.sub.1, a.sub. 2, . . . a.sub.1152 to form corresponding ones of the data b.sub.1, b.sub.2, . . . b.sub.768 For example, pixel b.sub.2 is obtained by calculating an average of the pixels a.sub.2 and a.sub.3. Other interpolation formulae may instead be employed; for example, pixel b.sub.2 may be derived instead as follows: EQU b.sub.2 =(a.sub.1 +3a.sub.2 +3a.sub.3 +a.sub.4)/8
It will be appreciated that in order to carry out the necessary conversion of the video signals generated by the upper level system for reproduction by the lower level system as described above, a signal processing system must be provided having a memory capability in order to carry out the interpolation process. This results in increased complexity and cost of the lower level system.